Head First Java

Question 1

What should the first line of your program be in the main class?

Answer 1

public static void main (String[] args){

}

Question 2

What are the 3 standard loop constructs in Java?

Answer 2

while, do while and for

Question 3

How do you declare and give initial values to a three-element String array in Jave?

Answer 3

String[] pets = {“Fido”, “Zeus”, “Bin”};

Question 4

What does random() return in Java? How do you use this?

Answer 4

It returns a number between 0 and just less than one. You use it by multiplying the max amount of your desired range by whatever random return to give you your random number.

Question 5

In terms of adaptability, what is one of the biggest advantages of OOP?

Answer 5

You can accommodate new requirements without altering previously tested, delivered, and accepted code. There is no need to recompile the existing code.

Question 6

What does the dot operator do in respect of a class?

Answer 6

It gives you access to the objects state and behavior i.e. the instance variables and the methods,

Question 7

What’s the function of the main method? Why

Answer 7

It serves to test your classes and to start your program off. The real idea of OOP is to get classes talking to classes.

Question 8

How do you write a global variable called totalValue in Java?

Answer 8

You don’t. There is no concept of global variables in Java.

Question 9

What is a manfest text file?

Answer 9

It is a text file bundled with your application classes that defines which class in the zipped jat file holds the main method.

Question 10

What are 2 types of variable?

Answer 10

Primitive and Reference

Question 11

What are the four integer primitives in Java?

Answer 11

long, int, short, byte

Question 12

What are the two floating-point primitives in Java?

Answer 12

float and double

Question 13

What are the six types of integer and floating-point variables in Java?

Answer 13

float double

byte short int long

Question 14

What values can be held in a byte, short, integer, and long?

Answer 14

Byte:c. -127 to 127
Short: c.-31k to 31k
Int: c. -2bn to 2bn
long - more than 2bn

Question 15

How many primitive types are there in Java?

Answer 15

8

Question 16

Name the primitive variable types in Java

Answer 16

char, bool, byte, short, int, long, float, double

Question 17

What does an object variable contain?

Answer 17

The address of the object - a reference to the place in memory

Question 18

Declare an empty integer array

Declare an integer array with 7 empty elements

Answer 18

int[] a

a = new int[7]

Question 19

Declare an array of Dog objects called pets

Answer 19

Dog[] pets;

Question 20

Declare an array of Dog objects called pets of length 7. What do we need to do to use the array?

Answer 20

Dog[] pets ;
pets = new Dog[7];
We have declared an array of 7 elements that will hold dog objects but, we have not declared any dogs to go into the array elements.

we could say e.g. pets[0] - new Dog();

Question 21

If we have an array which holds Dog objects (array called myDogs), how do we access methods of elements in the aray

Answer 21

myDogs[0].bark();

Question 22

Is it ok not to initialize local and instance variables?

Answer 22

Instance variables - yes, because they will always get a default value of 0, false or null. Local variables, no. You must initialize them before use.

Question 23

What’s the code to convert a string to an integer?

Answer 23

Integer.parseInt(“3”);

Question 24

Write a for each loop

Answer 24

for(int number : listOfNumbers){

}

Question 25

What does break; do?

Answer 25

Get’s you out of a loop immediately

Question 26

When we write test code, how do we decide what to test?

Answer 26

We test the important stuff in the method/class i.e. anything non trivial so e.g. a simple setter method wont need testing.

Question 27

Broadly, there are 7 steps to coding a method, what are they?

Answer 27

1. Identity what it’s supposed to do - what goes in what comes out, how it gets from one to the other(the algorithm)
2. List the class instance variables
3. Write the prep code/pseudo-code
4. Write the test code for the method
5. Implement the method
6. Test
7. Debug

Question 28

We have an array myArray which has elements {1, 2, 3}. How do we add another element 7?

Answer 28

We can’t as arrays are fixed in size. An ArrayList is a better choice - an array that can vary in size as required.

Question 29

A subclass extends the superclass - what is this describing

Answer 29

Inheritance

Question 30

What does a subclass actually inherit from the superclass?

Answer 30

The instance variables and the methods

Question 31

How do you apply the IS-A test in inheritance?

Answer 31

If a class extends another class i.e. is a subclass of the superclass then we can say subclass IS-A superclass e.g. cat IS-A feline, cola IS-A drink. If you can’t do the IS-A test then you have got something wrong

Question 32

Where you have multiple methods in an inheritance tree e.g. doSomething(), which is invoked in a method call?

Answer 32

The lowest one in the tree

Question 33

How does a superclass control what a subclass gets access to or not in terms of variables and methods?

Answer 33

It uses public and private. Stuff that is marked private will not be inherited.

Question 34

We have supertype Animal and sub types cat, dog, hippo. How can we write efficient code that gets each of the animals to execute their own eat method?

Answer 34

We can use polymorphism. Declare an array of type Animal (call it e.g. animals)and put in there an object of type cat, dog and hippo. Iterate through the array and then say animals[i].eat(). The compiler will then access the correct eat method for each of the animals.

Question 35

Is it possible to declare a method parameter as a super class type? Why would we do this?

Answer 35

Yes. It is so that we can pass in objects of any of the sub class types and have methods appropriate to that sub class executed. People will also be able to add new subtypes and these methods will still work.

Question 36

If you have used inheritance but you absolutely want to make sure that no one sub classes and overrides a particular method - how do you do this?

Answer 36

You can mark that method as final in the super class. This prevents it from being overridden and ensures it works exactly the way you intended it to.

Question 37

What is an abstract class?

Answer 37

A class that cannot be instantiated.

Question 38

Why do we need abstract classes?

Answer 38

We do not want the class to be instantiated e.g. if we have an animal super class we can say Animal ani = new Animal() but this doesn’t make sense - what exactly does this animal look like? We don’t know- the intention was to create sub classes which borrow behaviour from this class and were flexible enough to have their own behaviour but the main super class was never intended for instantiation.

Question 39

How do we make a class called Canine that extends Animal an abstract class?

Answer 39

We add the word abstract before the class declaration. You can say:
Canine c = new Dog(); but you can't say
Canine c = new Canine()